System and method for measuring battery current

ABSTRACT

Systems, devices and methods are provided for measuring battery current. According to one aspect, a medical device is provided that comprises a battery, a pulse generator, and a current measuring device. The pulse generator draws a pulse generator current from the power source, and the current measuring device determines the pulse generator current or tracks charge depletion from the battery. The current measuring device comprises an oscillator and a counter. The oscillator produces an oscillating output with a frequency of oscillation dependent on the pulse generator current, and the counter provides an oscillation count for the oscillating output. The current measuring device is capable of being calibrated while continuously determining the pulse generator current. In one embodiment, the current measuring device includes at least two current sources, each including an operational amplifier that has an autozeroing feature.

CROSS-REFERENCE TO RELATED APPLICATION(s)

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/829,825, filed on Apr. 10, 2001, the specification of whichis incorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention relates generally to the field of medical devices,and more particularly to systems, devices, methods and applications formeasuring current drain or charge depletion for medical devices.

BACKGROUND

[0003] Medical devices, including cardiac stimulus devices such asimplantable cardiac pacemakers and implantable cardioverterdefibrillators (ICDs), are surgically implanted within a patient.Cardiac stimulus devices have one or more electrical leads with one ormore electrodes that conduct signals to and receive signals from thepatient's heart. As such, cardiac stimulus devices provide electricaltherapy to the patient. A programming device or programmer is able tocommunicate with the medical device through a communication link. Thecommunication link provides means for commands and data to betransmitted and received between the programmer and the device.

[0004] Longevity is an important aspect regarding the performance of themedical device, and it is determined primarily by the capacity of thebattery and the current drain of the electronics; i.e. battery status.The current drain depends on a number of variables, including theimpedance(s) of the pacing lead(s), the ratio of sensed beats to pacedbeats, the background current of the device's electronics, and thecharacteristics of the pacing pulse(s) such as pacing rate, pacing mode,amplitude and width. Approximations for current drain and/or chargedepletion for the medical device are obtained by estimating thesevariables. However, estimating these variables is difficult because theyvary over the lifetime of the device. Tradeoffs that sacrifice thelongevity of the medical device for the safety of the patient may berequired to maintain an appropriate safety margin that accounts for thegranularity and uncertainty in estimating these variables.

[0005] Latent electrical faults within the implanted device may causeelevated current drains. These electrical faults can go undetected earlyon in the life of an implanted device without a means for measuring andreporting the current of the implanted device. A problem with reportingcurrent measurements for an implanted device is that the communicationsystem, such as a telemetry circuit, draws considerably more currentduring a communication session than in a quiescent period. This extracurrent drawn by the telemetry circuit can adversely affect the currentmeasurements.

[0006] Therefore, there is a need in the art to provide a system andmethod for measuring the current drain or charge depletion forimplantable medical devices, and to provide applications for determiningthe battery status, detecting electrical faults, and reporting thebattery current.

SUMMARY OF THE INVENTION

[0007] The present subject matter addresses the aforementioned problemsby providing systems, devices and methods for measuring current drainand tracking the charge depletion. Applications include a method fordetermining battery status, for determining potential faults, and fordisplaying or reporting current drain.

[0008] One aspect provides a medical device, comprising a battery, apulse generator, an electrode system, and a current measuring device.The pulse generator draws a pulse generator current from the battery.The electrode system is in electrical communication with the pulsegenerator and applies electrical therapy, such as pacing pulses, to apatient. The current measuring device determines the pulse generatorcurrent, and is capable of being calibrated as it is determining thepulse generator current. Therefore, the current measuring device is ableto continuously measure or track the current drain from the battery andstill be calibrated or periodically calibrated to maintain the accuracyof these current measurements.

[0009] One embodiment of the current measuring device comprises anoscillator and a counter. The oscillator produces an oscillating outputthat has a frequency of oscillation dependent on the pulse generatorcurrent, and the counter provides an oscillation count for theoscillating output, i.e. the counter counts the number of oscillationsin the oscillation output over a known time interval. The pulsegenerator current is determined from the oscillation count.

[0010] One embodiment of the current measurement device comprises acurrent source that provides an oscillator input current that isdependent on the pulse generator current to the oscillator. One currentsource embodiment includes a current divider for dividing current drawnfrom the battery into the pulse generator current and an attenuatedcurrent. The current divider includes a sense resistor, an attenuationresistor, and an operational amplifier. The pulse generator currentflows through the sense resistor, and the attenuated current flowsthrough the attenuation resistor. The operational amplifier appliesnegative feedback to provide a voltage drop across the attenuationresistor that is substantially equal to the voltage across the senseresistor.

[0011] In another embodiment, the device also includes a second currentsource having a second operational amplifier. Both operationalamplifiers have an autozeroing feature that compensates for an offsetvoltage that could adversely affect the value of the attenuated current,and thus adversely affect the determination of the pulse generatorcurrent. The pulse generator current is continuously measured using oneof the two current sources. As one current source autozeros, the othercurrent source continues to operate.

[0012] Another aspect provides a method for determining current drainfor a medical device, wherein the oscillations are counted for a periodof time, and the pulse generator current is determined by theoscillation count over the period of time. Another aspect provides amethod for tracking charge depletion of a battery in an implantablemedical device, wherein the charge depletion of a battery is determinedby continuously summing the oscillation count over successive periods oftime.

[0013] These and other aspects, features, embodiments, applications, andadvantages of the invention will become apparent from the followingdescription of the preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a schematic view of one system embodiment.

[0015]FIG. 2 is a block diagram for one medical device embodiment.

[0016]FIG. 3 is a block diagram of the electronic circuitry for themedical device of FIG. 2.

[0017]FIG. 4 is a block diagram of the electronic circuitry for themedical device of FIG. 2.

[0018]FIG. 5 is a schematic of the electronic circuitry according to theblock diagrams of FIGS. 3 and 4.

[0019]FIG. 6 is a block diagram of the electronic circuitry for themedical device of FIG. 2.

[0020]FIG. 7 is a schematic of the electronic circuitry according to theblock diagram of FIG. 6.

[0021]FIG. 8 is a schematic of an operational amplifier that has anautozero feature such as those that may be used in the electroniccircuitry of FIG. 7.

[0022]FIG. 9 is a block diagram of one embodiment of the electroniccircuitry for the medical device of FIG. 2, wherein the currentmeasuring device does not measure current drawn by the telemetrycircuitry during telemetry sessions.

DETAILED DESCRIPTION

[0023] The present subject matter addresses the aforementioned problems,and provides systems, devices and methods for measuring battery currentand tracking charge depletion of a battery. In the following detaileddescription, references are made to the accompanying drawings thatillustrate specific embodiments in which the invention may be practiced.In the drawings, like numerals describe substantially similar componentsthroughout the several views. Changes in the electrical, mechanical,structural, logical or programming designs may be made to theembodiments without departing from the spirit and scope of the presentinvention. The following detailed description is, therefore, not to betaken in a limiting sense and the scope of the present invention isdefined by the appended claims and their equivalents.

[0024]FIG. 1 illustrates a cardiac rhythm management system 100, whichgenerally comprises a medical device 102 and a programmer device 104capable of communicating with the medical device 102 through acommunication channel 106 such as that which may be created usinginduction coils or radio frequency telemetry, for example. The medicaldevice 102 has an electrode system 108 for providing therapy; i.e. fordelivering electrical pulses produced by the pulse generator to apatient's heart 110 and for sensing intrinsic electrical pulses producedby the patient's heart 110. Commands and data, including both raw andprocessed data, are capable of being communicated between the medicaldevice 102 and programmer device 104. Raw data is processed usingcircuitry, software programs, or a combination thereof, in the medicaldevice 102, in the programmer device 104, or in both devices.

[0025]FIG. 2 shows a medical device 202 that generally comprises a powersource 212 such as a battery, a pulse generator 214, and a currentmeasuring device or battery current measurement circuitry 216. The pulsegenerator 214 draws a pulse generator current from the power source 212,and the current measuring device 216 determines the pulse generatorcurrent. In one embodiment, the current measurement device 216determines the average pulse generator current over a period of time. Inanother embodiment, the current measuring device 216 determines thetotal charge depletion of the power source 212. In other embodiments,the current measuring device 216 is implemented in applications ormethods for determining battery status, for detecting potential faults,and applications for displaying current drain. These applications aredescribed below.

[0026] As indicated in FIG. 2, one embodiment of the pulse generator 214comprises a processor 218, a memory portion 220 operably connected tothe processor 218, a pulse and sense portion 222 operably connected tothe processor 218 for delivering and receiving electrical pulses to andfrom the heart 210, a regulated voltage supply 224, and a communicationportion 226 operably connected to the processor 218 for communicatingwith a programmer device 104. The pulse generator 214 forms a pulsegenerator load that draws a pulse generator current from the battery212. The current measuring device 216 provides means for determining thepulse generator current, which in one embodiment includes means fordetermining the average pulse generator current, and in anotherembodiment includes means for tracking charge depletion in the battery212.

[0027]FIG. 3 illustrates electronic circuits for the medical device 302and shows one embodiment of the current measuring device 316 thatincludes an oscillator 328 and a counter 330. The oscillator 328produces an oscillating output 332, which has a frequency of oscillationdependent on the pulse generator current I_(PG) that the pulse generatorload 314 draws from the battery 312. This frequency of oscillation isrepresented by the equation: f_(osc)=k×I_(PG). The counter 330 providesor determines an oscillation count for the oscillating output 332, i.e.the counter 330 counts the oscillations of the oscillation output 332.The pulse generator current I_(PG) is determined from the oscillationcount through a data process that may occur in the medical device, theprogrammer device, partially in both devices, or even in anotherprocessing device. Thus, the oscillator 328 provides means for providingan oscillating output 332 with a frequency of oscillation that isdependent on the pulse generator current I_(PG), and the counter 330provides means for providing or determining an oscillation count for theoscillating output 332.

[0028] One embodiment of the current measuring device 316 furtherincludes a comparator 340. The comparator 340 receives the oscillatingoutput 332 from the oscillator 328 and generates a correspondingoscillating comparator output 342 that is received by the counter 330.

[0029] One embodiment of the medical device 302, as illustrated in FIG.3, includes a current source 344 that provides an oscillator inputcurrent 338 for the oscillator 328. This oscillator input current 338 isdependent on the pulse generator current I_(PG), and thus provides meansfor providing an oscillator input current 338 to the oscillator 328 thatis dependent on the pulse generator current I_(PG). FIG. 3 shows thatthe current source 344 provides an oscillator input current 338 that isproportional to the pulse generator current I_(PG), i.e. “A×I_(PG)”.

[0030]FIG. 4 illustrates electronic circuitry for the medical device 402that includes the battery 412, the pulse generator load 414 and thecurrent measuring device 416. FIG. 4 shows one embodiment of the currentsource 444 that includes a current divider 446 for dividing currentdrawn from the battery 412 into the pulse generator current I_(PG), andan attenuated current 448. The attenuated current 448 is small enough soas not to substantially increase the current drain from the battery andthus the life of the medical device, and so as not to adversely affectthe operation of the medical device by limiting the pulse generatorcurrent I_(PG). One embodiment of the current source 444 that alsoincludes current mirror stages 452 that further reduce the current asdesired for the characteristics of a particular oscillator 428. FIG. 4also shows that the current measuring device 416 includes an oscillator428, a comparator 440 and a counter 430, which were previously describedwith respect to FIG. 3.

[0031]FIG. 5 is a schematic of the electronic circuitry according to theblock diagram of FIGS. 3 and 4. One embodiment of the current divider546 includes a sense resistor R1 through which the pulse generatorcurrent I_(PG), flows, an attenuation resistor R2 through which theattenuated current 548 flows, and an operational amplifier 550 whichapplies negative feedback to provide a voltage drop across theattenuation resistor R2 that is substantially equal to the voltageacross the sense resistor R1.

[0032] The current mirror stages 552 include a first current mirrorstage 554 for receiving the attenuated current 548 and a second currentmirror stage 556 for providing the oscillator input current 538. Thefirst current mirror stage 554 provides an additional M-to-1 currentreduction and the second current mirror stage 556 provides an additionalN-to-1 current reduction in the oscillation input current 538.Therefore, the current mirrors 554 and 556 further attenuate the signalby a factor of M×N as desired for the oscillating characteristics of thechosen oscillator 528. Thus, in the illustrated embodiment that includesboth the current divider 546 and the current mirrors 552, the oscillatorinput current 538 is represented by the following equation.

Oscillator Input Current=A×I _(PG)=[(R1)/(R2 ×M×N)]×I _(PG)

[0033] One embodiment of the oscillator 528 comprises a capacitor 534, areset switch 536, and a comparator 540. The capacitor 534 is charged byan oscillator input current 538 that is dependent on the pulse generatorcurrent I_(PG). The reset switch 536 discharges the capacitor 534 whenthe voltage across the capacitor 534 increases to a level approximatelyequal to a reference voltage V_(REF). The comparator 540 compares avoltage across the capacitor 534 with the reference voltage V_(REF), andprovides a resulting oscillating comparator output 542 to the counter530.

[0034] The counter 530, in one embodiment, is capable of beingperiodically reset using a reset strobe, represented by the RESET—Ninput line, that is derived from a known, stable time base such as acrystal oscillator. As will be discussed in more detail below by way ofexample, resetting the counter 530 provides means for collectingoscillation counts over one or more periods of time to determine thepulse generator current, or for collecting oscillation counts over thelifetime of a battery 512 to track the charge depletion of the battery512.

[0035] For the illustrated oscillator 528, the frequency of theoscillating output 532, and thus of the oscillating comparator output542, is determined by the following equations:

f _(osc)=(Oscillator Input Current)/(C _(osc) ×V _(ref));

f _(osc)=(A*I _(PG))/(C _(osc) ×V _(ref)); and

f _(osc) I _(PG)×[(R1)/((C _(osc) ×V _(ref))×(M×N×R2))].

[0036] For example, if I_(PG) is 10 micro amperes, R1 is 250 ohms, R2 is250k ohms, V_(REF) is one volt, C_(OSC) is 50 picofarads, and M×N=20,the output frequency of the oscillator is 10 Hz. These values providethe circuit with a convenient 1 microampere per Hertz scale factor. Thecounter 530 measures the output frequency by counting the oscillationsfrom the comparator 540 for a time period. The digital counter can bereset periodically with RESET—N, which is generated from a known, stabletime base (i.e. a crystal oscillator).

[0037] The above-described circuit provides the medical device 502 withmeans for determining the pulse generator current I_(PG):

I _(PG) f _(osc)×[((C _(osc) ×V _(ref))×(M×N×R2))/(R1)]

[0038] Through data processing, the average pulse generator current overa period of time and the total charge depleted from the power source maybe determined from the number of counted oscillations. This dataprocessing is performed by circuitry, software programs, or acombination thereof in the medical device, the programmer device, acombination of the medical device and the programmer, or anotherprocessor.

[0039] The pulse generator current I_(PG) is measured by placing thecurrent sense resister R1 in series with the battery 512. Drawing thepulse generator current I_(PG) through this sense resistor R1 results ina small voltage drop, V_(SENSE). The resistance value of the currentsense resistor R1 should be small enough so that it does not impededevice performance, yet should be large enough so that the typicalvoltage drop V_(SENSE) which appears across the resistor R1 can beeasily measured or otherwise detected. Although a range of values may beused, a 250 ohm sense resistor R1 is suitable for a pulse generator load514 that draws an average of 10 to 100 microamperes. This voltage dropis reproduced across a larger-valued resistor R2, such as a 250k ohmresistor for example, to generate in the attenuated current 548. Thevoltage across the resistor R2 is substantially the same as the voltageacross the resistor R1 due to the feedback applied by the operationalamplifier 550 and transistor 555. Because of the larger resistance ofthe resistor R2, a smaller or attenuated current 548 flows through theresistor R2. The resulting current 548 is further attenuated by a factorof M×N using the two current mirror stages 554 and 556 to form theoscillator input current 538.

[0040] The oscillator input current 538 charges the capacitor 534 in acyclic fashion from zero volts up to the reference voltage V_(REF). Acomparator 540 monitors the voltage across the capacitor 534 anddischarges the capacitor 534 back to zero volts once the voltage reachesor approximately equals V_(REF).

[0041] As provided earlier, the current source 544 includes anoperational amplifier 550. The operational amplifier 550 should havesufficient dynamic range to accommodate the peak currents associatedwith the switched-mode power converters used in implantable devices.Furthermore, the operational amplifier 550 and attenuation circuitryshould have sufficient bandwidth to track the rapid changes in currentdrawn by the medical device. Connecting a large bypass capacitor acrossthe pulse generator load 514 serves to smooth out the current drawndirectly from the battery 512, which effectively reduces the magnitudeof current peaks and also spreads them out over time. Since both thevoltage drop across R1 and the input offset voltage errors for anoperational amplifier 550 can be on the order of a few millivolts, anoperational amplifier offset voltage can cause substantial differencesin voltages across resistors R1 and R2, and thus can introduceunacceptably large errors in the current measurement.

[0042]FIG. 6 shows a block diagram of another electronic circuitryembodiment for the medical device 602 of FIG. 2 adapted for controllingthe offset (i.e. autozeroing). In particular, the block diagram showsthat two current sources 644 a and 644 b are being used in the device616 for measuring pulse generator current, wherein one of the currentsources continues to operate in a fashion that allows the currentmeasurement device 616 to continuously measure current as the othercurrent source is calibrated. For example, in the case for currentsources that incorporate operational amplifiers, one of the operationalamplifiers may be autozeroed while the other operates normally. As shownin FIG. 6, separate oscillators 628 a and 628 b may be used inconjunction with the separate current sources 644 a and 644 b. Theswitches 664 and 666 cooperate to control whether the first circuit(S=1) or the second circuit (S=2) is measuring current.

[0043]FIG. 7 shows a schematic of an electronic circuitry embodimentaccording to the block diagram of FIG. 6. The autozeroed currentmeasurement circuit attempts to eliminate any operational amplifieroffset voltage errors from the current measurement. Since both thevoltage drop across R1 and the input offset voltage errors for anoperational amplifier can be on the order of a few millivolts, anoperational amplifier offset voltage can cause substantial differencesin voltages across resistors R1 and R2, and thus can introduceunacceptably large errors in the current measurement.

[0044]FIG. 7 shows two complementary circuits. Each one of theseillustrated circuits includes a current source 744 a and 744 b with anoperational amplifier 750 a and 750 b, and also includes an oscillator728 a and 728 b. The operational amplifiers 750 a and 750 b have anautozeroing feature, as will be discussed in more detail below withrespect to FIG. 8. Since an autozeroing or calibration function requirestime to perform, a current measuring device 716 having a single currentsource cannot autozero, or otherwise calibrate, the current source whilemeasuring the pulse generator current I_(PG). Using two operationalamplifiers 750 a and 750 b allows the device 716 to calibrate itself andstill continuously measure current. For example, while the operationalamplifier 750 a in one of the current sources 744 a is autozeroing tocalibrate the current measurement device 716, the other current source744 b will continue to provide an oscillator input current 738 b that isdependent on the pulse generator current I_(PG). Thus, the circuitcompensates for the offset voltages of the operational amplifiers 750 aand 750 b, and still continuously determines the pulse generator currentI_(PG).

[0045] The digital logic elements that appear on the right side of FIG.7 are a D flip-flop 758, a non-overlapping clock generator 760, and apair of inverters 762. The D flip-flop 758 acts as a toggle flip-flopthat flips to the opposite logic state on every rising edge of the clockinput. The non-overlapping clock generator 760 provides twocomplementary “non-overlapping active-high” outputs; i.e. provides twooutput signals (Q1 and Q2) that are essentially complementary but arenever high at the same time. The inverters 762 generate complementaryoutputs. Thus, the digital logic elements provide Q1, Q1—N, Q2, and Q2—Nthat are used to control the overall functioning of the above-describedcircuits or circuit branches, wherein the “—N” suffix indicates that thelogic signal is “active-low” and is used to turn on a PMOS device. WhenQ1 is high, the first current source 744 a and oscillator circuit 728 aare operating in a fashion to actively measure current, and theoperational amplifier 750 b in the second current source 744 b isautozeroed. When Q2 is high, the second current source 744 b andoscillator circuit 728 b actively measures current, and the operationalamplifier 750 a in the first current source 744 a is autozeroed.

[0046] Thus Q1, including the signals Q1 and Q1—N, generally representsa first state in which the first current source operates to measurecurrent while the second current source is calibrated or autozeroed. Q2,including the signals Q2 and Q2—N, generally represents a second statein which the second current source operates to measure current while thefirst current source is calibrated or autozeroed. The circuit switchesbetween these states, as previously illustrated by switch 664 and switch666 of FIG. 6. FIG. 7 illustrates that switch 664 of FIG. 6 generally isformed by transistors 770 a and 770 b; and further illustrates thatswitch 666 of FIG. 6 generally is formed by transistors 772 a and 772 b.Transistors 768 a and 768 b, when actuated, effectively tie together theinputs of their respective operational amplifiers 750 a and 750 b forthe autozeroing process.

[0047]FIG. 8 represents an operational amplifier with an autozeroingfeature. The illustrated operational amplifier 850 is a two-stage,Miller-compensated operational amplifier with an autozero function. Thecapacitor “CCOM” provides stability. The first stage comprises thedifferential pair of transistors Q1 and Q2, which is supplied a biascurrent from transistor Q5, and which is actively loaded by the currentmirror formed by the transistors Q3 and Q4. The second stage comprisesthe transistor Q7, which is actively loaded with the current-sourcetransistor Q6.

[0048] The operational amplifier performs its autozero function when the“AZ” input is high, i.e. at logic 1. The autozero function is describedbelow with respect to the operational amplifier schematic of FIG. 8. Theinputs to the amplifier are effectively tied together by a PMOStransistor 768 a or 768 b, as illustrated in FIG. 7, which are activatedwhen the corresponding circuit is autozeroed. Both of the operationalamplifier inputs are capacitively coupled. The voltage V1 at the gate ofQ1 is at a reference voltage VR through an actuated transistor Q8, andthe offset voltage V2 of the first stage is sampled onto capacitor CIN1through an actuated transistor Q9. The capacitor CAZ capacitivelycouples the output of the first stage V3 to the input of the secondstage V4. Transistor Q10 is actuated by the high AZ signal and causesthe second stage transistor Q7 to be configured as an NMOS diode, which“self-biases” the second stage during autozeroing. After the “AZ” inputgoes low, i.e. at logic 0, the capacitor CAZ maintains the voltage V3-V4that it sampled while the AZ input was held high.

[0049] The figures presented and described in detail above are similarlyuseful in describing the method aspects of the present subject matter.The methods described below are nonexclusive as other methods may beunderstood from the specification and the figures described above.

[0050] One aspect of the present subject matter provides a method fordetermining current drain for a medical device. In this method, anoscillating output is provided with a frequency of oscillation that isdependent on a pulse generator current I_(PG). An oscillation count isprovided or determined for the oscillating output, and the pulsegenerator current I_(PG) is determined based on the oscillation countover the period of time. In one embodiment, the oscillating output isprovided by charging a capacitor, comparing a voltage formed across thecapacitor with a reference voltage V_(REF), and discharging thecapacitor when the voltage across the capacitor increases to a levelequal to the reference voltage V_(REF). In another embodiment, thismethod also includes providing an oscillator input current to theoscillator that is dependent on the pulse generator current I_(PG). Theoscillator input current is provided to the oscillator by dividingcurrent drawn from a battery into the pulse generator current I_(PG) andinto an attenuated current that is dependent on the pulse generatorcurrent I_(PG). The attenuated current is reduced to the oscillatorinput current.

[0051] Another aspect provides a method for tracking charge depletion ofa battery or other power supply in an implantable medical device. Inthis method, an oscillating output is provided with a frequency ofoscillation that is dependent on a pulse generator current I_(PG). Anoscillation count is determined or provided for the oscillating outputover a period of time. The charge depletion of a battery is determinedby continuously summing the oscillation count over successive periods oftime. In one embodiment, the oscillating output is provided by charginga capacitor, comparing a voltage formed across the capacitor with areference voltage V_(REF), and discharging the capacitor when thevoltage across the capacitor increases to a level equal to the referencevoltage V_(REF). In another embodiment, the method also includesproviding an oscillator input current that is dependent on the pulsegenerator current I_(PG). In another embodiment, the oscillator inputcurrent is provided to the oscillator by dividing current drawn from abattery into the pulse generator current I_(PG) and into an attenuatedcurrent that is dependent on the pulse generator current I_(PG), andreduces the attenuated current to the oscillator input current.

[0052] The system and method for measuring battery current, describedherein, is incorporated into several applications. These applicationsinclude, but are not limited to, the following applications fordetermining battery status, for detecting potential faults, and fordisplaying current drain.

[0053] Battery Status

[0054] The battery current measurement device described above generatespulses, wherein each pulse represents a quantity of charge given by theequation:

Q=[(C _(osc) ×V _(ref))×(M×N×R2)]/R1.

[0055] If the pacemaker battery has a 1 Amp Hour capacity, it holds acharge of 3600 C; i.e. 1 A×60 min/hr×60 sec/min=3600 C. If, as in theexample provided above, R1 is 250 ohms, R2 is 250k ohms, V_(REF) is onevolt, C_(OSC) is 50 picofarads, and M×N=20, each counter pulserepresents 1 μC. of charge. Therefore, in this example, the countershould count to 3.6×10⁹, which corresponds to (3600 C)×(1×10⁶ pulses/C).A 32-bit counter is appropriate as it corresponds to a value of2³²=4.294967296×10⁹. The degree of battery depletion can then bedetermined directly from this count value by multiplying the count bythe 1 μC of charge contained in the capacitor.

[0056] However, it is unlikely that the charge depleted from the batteryneeds to be tracked to within 1 μC. Rather, only N of the mostsignificant bits are needed to determine the charge count, wherein thevalue of N depends on the degree of precision desired for the batterydepletion estimate. For example, to report the battery depletion as apercentage with 1% resolution, only the seven (N=7) most significantbits (2⁷⁼128) of the charge counter need to be examined, whichcorresponds to a resolution of:${\frac{1}{128} \times \frac{2^{32}}{3.6 \times 10^{9}}} = {0.93\quad \%}$

[0057] A designer, with a view toward ensuring adequate safety marginsthat maintain acceptable device performance, selects battery depletionvalues for the pacemaker elective-replacement-time (ERT) and theend-of-life (EOL) indications. The battery depletion values for ERT andEOL may be viewed as a replacement percentage.

[0058] Fault Detection

[0059] Latent electrical faults within implanted pacemakers can oftenresult in elevated battery current drain. Unfortunately, these types offailures can go undetected early on in the life of a device if a batterycurrent measurement circuit is not available. To facilitate detection ofthese failures, the battery current measurement device described abovecan be employed.

[0060] An appropriate upper limit for the battery current, i.e. faultdetection criterion, is determined in order to identify battery currentdrain that is excessive and thus indicative of an electrical faultwithin the device. For example, an embodiment that derives this upperlimit from a worst-case estimate of the battery current drain over allpossible operating conditions can result in an under-sensitive currentfault detector. However, an embodiment that derives the upper limit froman “expected current drain” value is more sensitive to fault detection.The upper limit is compared against the actual current measurement valueto determine whether a current fault exists. The “expected currentdrain” value can take into consideration the pace amplitude, the pulsewidth, the lead impedance, the pacing rate, and the paced-to-intrinsicratio. A current fault is declared when:

I _(MEASURED)>(I _(EXPECTED) ×K)+I _(OFFSET.)

[0061] For example, if it is desired to declare a fault when themeasured current value is in excess of twice the expected current value,K is set to 2 and I_(OFFSET) is set to 0 μA. As another example, if itis desired to declare a fault when the measured current value exceedsthe expected current value by 50 μA, K is set to 1 and I_(OFFSET) is setto 50 μA.

[0062] The current measurement can be obtained once a fault detectioncriterion has been established. As described previously, the batterycurrent measuring device generates a digital output pulse signal whosefrequency is proportional to the battery current drain. The frequency ofthis pulse signal, and hence the battery current, is determined bycounting the number of pulses that appear over a known time interval.

[0063] The time interval used to determine the frequency is chosen toprovide optimal fault detection performance. Choosing a relatively largemeasurement smooths the normal minimum and maximum current extremes toan acceptable level. This time averaging is desirable because a batterycurrent fault should not be triggered for the large current transientsthat are a normal part of operation for medical devices. In a pacemaker,for example, a fault should not be triggered when the pacing supplyrecharges immediately following the delivery of a pace. However, if theinterval is too long, it becomes difficult or even impossible tolocalize intermittent fault conditions to a particular time.

[0064] Battery Current Reporting

[0065] The battery current measurement device is incorporated in anapplication for reporting the present current drain value back to thephysician or clinician who is performing a medical device implantprocedure or a follow-up evaluation. In order to obtain the currentmeasurements, the clinician interrogates the device using an externalprogrammer. The interrogation occurs through a communication channel,such as a telemetry channel. This telemetry activity typically requiressignificant battery current. Thus, an instantaneous or real-time currentmeasurement during a telemetry session includes this additionaltelemetry current which is not present in normal day-to-day pacemakeroperation.

[0066]FIG. 9 is a block diagram of one embodiment of the electroniccircuitry for the medical device of FIG. 2. To alleviate theabove-described problem, the current measurement device 916 does notmeasure current drawn by the telemetry circuitry 926 during telemetrysessions. FIG. 9 shows two switches 980 and 982, and further shows thatthe two switches have a first state S=1 and a second state S=2. Theelectronic circuitry is typically in the first state S=1; i.e. anon-telemetry session. The switch 980 is open and the switch 982 isclosed in the first state such that the quiescent current drawn by thetelemetry circuit is included in the current I_(PG). The switch 980 isclosed and the switch 982 is open in the second state such that thehigher current drawn by the telemetry circuit during a telemetry sessionis not included in the current I_(PG).

[0067] This application is intended to cover any adaptations orvariations of the present invention. It is manifestly intended that thisinvention be limited only by the claims and equivalents thereof.

What is claimed is:
 1. A battery-powered medical device, comprising:circuitry to be powered by and draw current from a battery to providemedical therapy; a current measurement circuit to measure the currentdrawn from the battery; and means to calibrate the current measuringcircuit when the current measurement circuit is measuring the currentdrawn from the battery.
 2. The device of claim 1, wherein: the currentmeasurement circuit includes means to select a first current source or asecond current source to charge an oscillator, wherein both the firstand second current sources provide an input current that is dependent onthe current drawn from the battery; and the means to calibrate thecurrent measuring circuit when the current measurement circuit ismeasuring the current drawn from the battery includes: means tocalibrate the second current source when the first current source isselected to charge the oscillator; and means to calibrate the firstcurrent source when the second current source is selected to charge theoscillator.
 3. The device of claim 2, wherein the means to calibrate thefirst current source includes means to autozero an offset voltage for afirst operational amplifier, and the means to calibrate the secondcurrent source includes means to autozero an offset voltage for a secondoperational amplifier.
 4. A medical device, comprising: a batteryterminal for connection to a battery; a pulse generator connected to thebattery terminal to draw a pulse generator current and generateelectrical therapy pulses; and a current measuring device to determinethe pulse generator current and to be calibrated while determining thepulse generator current.
 5. The medical device of claim 4, furthercomprising an electrode system in electrical communication with thepulse generator to deliver the electrical therapy pulses.
 6. The medicaldevice of claim 5, wherein the pulse generator comprises: a processor; amemory portion operably connected to the processor; a pulse and senseportion operably connected to the processor and to the electrode system;and a communication portion operably connected to the processor towirelessly communicate with a programmer.
 7. The medical device of claim4, wherein the current measuring device is adapted to produce anoscillating output with a frequency of oscillation that is dependent onthe pulse generator current and to provide an oscillation count fromwhich the pulse generator current is detennined.
 8. The medical deviceof claim 4, wherein the current measuring device is adapted to determinethe average pulse generator current over a period of time.
 9. Themedical device of claim 4, wherein the current measuring device isadapted to determine a total charge depleted from the battery.
 10. Themedical device of claim 4, wherein the current measurement device isadapted to compare a measured current drain against an expected currentdrain to detect potential faults.
 11. The medical device of claim 4,wherein: the current measurement device further comprises at least twocurrent sources to provide an oscillator input current; each of thecurrent sources has a calibration state and a current measurement state;and one of the at least two current sources is adapted to be in thecalibration state as the other of the at least two current sources isadapted to be in the current measurement state.
 12. The medical deviceof claim 11, wherein each of the at least two current sources includesan operational amplifier having a feature to autozero an offset voltage.13. A device for determining a pulse generator current drawn from abattery of an implantable medical device, comprising: a first means toprovide a first input current dependent on the pulse generator current;a second means to provide a second input current dependent on the pulsegenerator current; means to receive the first and second input currentand provide an output with a frequency of oscillation dependent on thepulse generator current; and means to count the frequency of oscillationof the output.
 14. The device of claim 13, wherein the first means toprovide a first input current and the second means to provide a secondinput current are similar such that the first input current and thesecond input current are interchangeable for use to provide an outputwith a frequency of oscillation dependent on the pulse generatorcurrent.
 15. A device for determining a pulse generator current drawnfrom a battery of an implantable medical device, comprising: a currentsource to provide an input current dependent on the pulse generatorcurrent, wherein the current source is adapted to be calibrated whileproviding the input current; an oscillator to receive the input currentand produce an oscillating output, wherein the oscillating output has afrequency of oscillation dependent on the pulse generator current; and acounter to provide an oscillation count for the oscillating output,wherein the pulse generator current is determined from the oscillationcount.
 16. The device of claim 15, wherein the current source includes acurrent divider to divide current drawn from the battery into the pulsegenerator current and an attenuated current, wherein the input currentis dependent on the attenuated current, the current divider including: asense resistor through which the pulse generator current flows; anattenuation resistor through which the attenuated current flows; and anoperational amplifier to provide substantially equal voltage dropsacross the sense resistor and across the attenuation resistor.
 17. Thedevice of claim 15, further comprising a second current source includinga second operational amplifier, wherein the operational amplifiers havea feature to autozero an offset voltage, and wherein one current sourceis adapted to autozero while the other current source is adapted toprovide the input current.
 18. The device of claim 15, wherein thecurrent source further includes at least one current mirror stage toperform a current reduction function.
 19. A method to measure current ina battery-operated medical device, comprising: selecting one of a firstcurrent source and a second current source to provide an input currentto charge an oscillator, wherein the input current is dependent on acurrent drawn from a battery; calibrating the second current source whenthe first current source provides the input current to charge theoscillator; counting a number of oscillations resulting from the inputcurrent provided by the first current source; calibrating the firstcurrent source when the second current source provides the input currentto charge the oscillator; and counting a number of oscillationsresulting from the input current provided by the second current source.20. The method of claim 19, wherein: selecting one of a first currentsource and a second current source to provide an input current to chargean oscillator includes selecting one of a first operational amplifierand a second operational amplifier to provide the input current;calibrating the second current source includes autozeroing an offsetvoltage of the second operation amplifier; and calibrating the firstcurrent source includes autozeroing an offset voltage of the firstoperational amplifier.